When examining the surface of metal bands, in particular metal bands consisting of Cu or Cu-alloys, it is often the case that indeed directives exist for the manufacture of the specimens to be examined, however, the result of the test is determined through a subjective comparison by the observer with a preset standard series so that the result of the test depends among others on the skill and the experience of the tester.
This is, for example, the case when evaluating the bending edge of metal bands.
Specimens for the bend test are manufactured corresponding to EN 7438:2000 using suitable bending punches and matrixes. The bend specimens are characterized by thickness s, width b, bend radius r, bend angle and bend direction relative to the rolling direction (see FIGS. 1 and 2) [compare Wieland-Handbuch “Kupferwerkstoffe” (6th Edition 1999, Pages 233–235), there: WR=rolling direction, BK=bending edge]. Usually specimens with a 90° or 180° bend angle are utilized. The specimen widths vary between 0.25 mm and 20 mm, the thickness extends from 0.10 mm to 2 mm, the bend radii assume values between 0 mm and 10 mm.
The ASTM Designation B 820-98 discloses directives for carrying out the testing. The bend samples are at present examined under the stereomicroscope (enlargement usually 10 times). A division into six classes takes place:
ClassEvaluation Text1smooth, no cracks2slightly rough, slight orange peel, no cracks3orange peel to strong orange peel, no cracks4starting to crack, incipient cracks5cracks to strong cracks6specimen broken
The occurrence of incipient cracks in the bending edge counts as error criterion.
The results are documented by indicating the bend radius at which a crack does not quite yet occur, together with the bend angle and bend direction. Since this method is not quantitative, the exactness of this test method with regard to the bending ability of the examined material cannot be indicated. The result of the method depends furthermore on the tester.